In an industrial process steam is often used as a driving force or as an inhibitor for all kinds of chemical or other processes.
Steam is generally generated in a boiler whose pressure and temperature are generally fixed.
The industrial process generally requires steam at a lower pressure and temperature than at the output of the boiler, whereby the desired steam conditions can also be variable.
Hence in most steam installations a pressure reducing valve is used between the boiler and the downstream industrial process that allows the steam to expand to the desired pressure required for the industrial process.
Generally saturated steam is used, which by definition does not contain any water in liquid form as all water present in the steam has evaporated into a gas.
It is known that with saturated steam there is an unequivocal link between the pressure and temperature of the steam. In other words, if the temperature of the steam is known, the pressure can also be determined from it and vice versa.
The pressure reducing valve is thereby opened or closed more or less to obtain a pressure that is equal to the pressure required by the downstream process. During expansion, the pressure and temperature of the steam change according to an isenthalpic law known in thermodynamics.
An advantage of such control is that it is very simple.
However, a disadvantage of such control is that the pressure drop is not used for an efficient conversion to another form of energy such as mechanical or electrical energy for example.
Another disadvantage is that it only enables the pressure to be controlled, whereby the isenthalpic expansion in the pressure reducing valve, starting with saturated steam, always supplies superheated steam at a temperature that is generally higher than desired. Superheating of the steam also means an inefficient exchange of heat in the downstream process and consequently must be limited as much as possible.
In order to reduce the temperature of the steam and the level of superheating, traditionally a boiler or ‘desuperheater’ is used that presents the disadvantage of being expensive and is consequently limited in its capacities.